The present invention relates to a method of operating electrolytic reduction cells for the production of aluminum.
In the Hall-Heroult process aluminum is produced by the passage of electric current through a molten electrolyte consisting of cryolite (Na.sub.3 AlF.sub.6) with normally an excess of AlF.sub.3 and small quantities of other alkali metal and alkaline earth metal fluorides such as LiF, CaF.sub.2 and MgF.sub.2 and containing dissolved alumina in an amount of about 2-8%. The cell is lined with carbon blocks which form the cathode and one or more carbon anodes are suspended above the cell and dip into the electrolyte.
The anodes may be of the pre-baked block type or the Soderberg type, in which a viscous carbonaceous mix is fed into a casing and is baked in situ.
In normal operation current passing between the anode and cathode decomposes alumina to form aluminium, which collects at the cathode, and oxygen, which is released at the anode and combines with the carbon anode to form gaseous oxides, which are freely ejected from under the anode face, because the carbon oxides do not wet the anode material.
In operating the electrolytic reduction cell the molten electrolyte is covered with a crust of solid material, onto which fresh alumina is supplied. Fresh alumina is supplied to the cell by breaking the crust and it is therefore not always possible to correctly gauge the amount of alumina that enters the electrolyte at each crust-breaking operation. In consequence occasionally the concentration of alumina in the cell electrolyte falls to a novel (0.5-2.2% alumina) where the fluoride salts start to decompose with consequent formation of gaseous fluorine compounds. These consist primarily of carbon tetrafluoride, which, unlike the carbon oxides, wet the anode material to form a stubborn, high-resistance film on the anode face and severely reduce the contact area between the face of the carbon anode and electrolyte. Under this condition, the overall cell voltage typically rises from 5 to 40 volts.
This phenomenon is normally referred to as "an anode effect". It is well known that corrective action must be taken quickly to counteract the deleterious results of the "anode effect" and regain normal operation of the cell. It is conventional to commence corrective action as soon as the cell voltage rises above 10 volts. In addition to restoring the alumina content to a normal operating level of 2-8%, positive action is required to remove the high resistance gas film at the anode face so as to reduce electrical resistance at the anode/electrolyte interface and to restore the current density at the interface to the normal operating level in the region of 0.55-1.10 amps/cm.sup.2.
In conventional practice when an anode effect is detected as a result of a sudden large rise in the cell operating voltage, the alumina concentration of the bath is restored by breaking the crust, and this is immediately followed by action to remove the layer of gaseous fluoride on the bottom face(s) of the anode(s) and to reduce the current density on the major portion thereof. For example, it is known to remove the gaseous film by scraping the anode face with a steel rake, by rapid injection of air into the inter-electrode space or by the insertion of a wooden pole under the anode. The last method depends on the rapid decomposition of the wood in contact with the bath electrolyte (circa 1000.degree. C.) with consequent release of large quantities of gas to flush the anode face. At the same time sufficient local disturbance in the metal pool is created to cause short circuiting of the metal to the anode face. This reduces the current density on the remainder of the anode face. Once the current density falls below a given critical value, the process is restarted.
The conventional methods of clearing "anode effects" are labour intensive and have other disadvantages. A significant quantity of materials, such as steel rakes or wooden poles is consumed with consequent introduction of impurities into the cell, and reoxidation of metal. Moreover these methods are virtually incapable of being performed under automatic control in response to rise in cell voltage.
Various methods of clearing anode effects, involving physical vertical movement of the anodes, have been devised. All electrolytic reduction cells are equipped with jacks for vertical movement of the anodes which are required to maintain the anode-cathode distance as nearly as possible at a target value, chosen to provide optimum cell operation. The consumption of anode material and the increase in the depth of the metal pool (the surface of which is the effective cathode surface) require periodic change in the anode face position to re-adjust the anode-cathode distance to the target value. Thus the cells are equipped with power-driven means for anode movement.
Existing methods of clearing anode effects by vertical movement of the anode involve some crust breaking action and increase of the alumina content of the bath. These methods have involved lowering the anode to bring the anode face into contact with the metal pool. The contact between the anode and the metal pool has the effect of displacing the fluoride gas film and at the same time short circuits the bath, thus reducing the current density on the remaining major portion of the anode face, which is out of contact with the molten metal. It is known that when the alumina content of the bath has been restored to a correct level and the process has been restarted by creating a local displacement of the fluoride gas film on the anode face and a local short circuit of the bath, the generated carbon oxides will flush away the remainder of the fluoride gas on the anode surface. This restores the cell to its normal operating condition.
Clearance of anode effects by anode lowering has been reasonably successful with electrolytic reduction cells of both the prebake-anode and horizontal-stud Soderberg type. In addition to reduction of current density on large areas of anode face, the method relies on replenishing and mixing alumina in the electrolyte bath through the tidal movement of the electrolyte in the peripheral region between the anode(s) and the cell wall resulting from the displacement of electrolyte as the anode(s) are first lowered and then raised.
That method of clearing anode effects can be initiated automatically in response to increase in cell voltage. Because of the high ratio of anode face area to bath surface area in the annulus between anode and cell side wall in a vertical stud Soderberg type cell, upward displacement of bath resulting from the lowering of the anode to make a short circuit would result in unacceptably large and frequent spillage of molten electrolyte. Furthermore the resulting movement of the electrolyte can lead to blockage of the gas collection skirt on the anode by frozen electrolyte.